There is a growing need within the industry for higher precision borehole surveys and for more frequent survey data. This is driven by several factors:
First, those of ordinary skill in the art will appreciate that there are the obvious increased financial benefits from optimal positioning of well bores within targets. Higher precision surveys also reduce the difficulty in reaching smaller targets. Further, there has been a recognized need for a more detailed understanding of the local structures of boreholes to control drag and to monitor deviation tendencies.
Geological steering (also known as geosteering) relies on prior well information and/or seismic information in addition to the geometric coordinates of the well being drilled. Since it takes on the order of a thousand feet to optimally position a well bore in a formation, geological markers, some well above the target zone, are typically used as reference points for programmed changes in the borehole trajectory. Any geometric error in the location of these points, such as errors caused by surveying inaccuracies, must be interpreted either as a change in geology or as a pure error. There is no known way to make a distinction between these alternatives. Hence, even though geosteering is used, geometric survey errors can cause a trajectory to completely miss the proposed target, or to be suboptimally located within that target. Positioning accuracy can be improved by taking more frequent surveys.
Historically, the conventional approach for borehole surveying was to take certain borehole parameter readings or surveys only when the drillstring was not rotating. While there were several reasons for taking measurement-while-drilling (MWD) measurements only in the absence of drillstring rotation, a principal reason for doing so was that the sensor arrays commonly used for measurement of the drillstring's azimuth and inclination (e.g., triaxial accelerometer and magnetometer sensor arrays) yielded the most reliable sensor outputs only when the drill string was stationary.
Some time ago, however, it came to be recognized that it is desirable in many circumstances to be able to measure azimuth and inclination while the drillstring is rotating. Examples of such circumstances include where drilling is particularly difficult and interruption of the rotation could increase drillstring sticking problems, or where knowledge of instantaneous bit walk information is desired in order to know and predict the real-time path of the borehole.
Those of ordinary skill will recognize that the prior art is replete with proposed systems and methods for obtaining azimuth and inclination measurements for the purposes of directional drilling. An early example is U.S. Pat. No. 4,733,733 to Bradley, titled “Method of Controlling the Direction of a Drill Bit in a Borehole,” which proposes utilizing a near-bit mechanics sensor and position monitor sensor to measure the magnitude of bending moments on the drill string.
It is more common, however, to utilize magnetometer and accelerometer sensor arrays disposed in a downhole segment of a drillstring to measure azimuth and inclination of a borehole. See, for example, U.S. Pat. No. 4,472,884 to Engebretson, titled “Borehole Azimuth Determination Using Magnetic Field Sensor.” See also, U.S. Pat. No. 4,813,274 to DiPersio et al., titled “Method for Measurement of Azimuth of a Borehole While Drilling;” U.S. Pat. No. 4,894,923 to Cobern et al., titled “Method and Apparatus for Measurement of Azimuth of a Borehole While Drilling;” U.S. Pat. No. 5,012,412 to Helm, titled “Method and Apparatus for Measurement of Azimuth of a Borehole While Drilling;” U.S. Pat. No. 5,128,867 to Helm, titled “Method and Apparatus for Determining Inclination Angle of a Borehole While Drilling;” U.S. Pat. No. 5,602,541 to Comeau et al., titled “System for Drilling Deviated Boreholes;” U.S. Pat. No. 6,405,808 to Edwards et al., titled “Method for Increasing the Efficiency of Drilling a Wellbore, Improving the Accuracy of its Borehole Trajectory and Reducing the Corresponding Computed [Ellipse] of Uncertainty;” U.S. Pat. No. 6,438,495 to Chau et al., titled “Method for Predicting the Directional Tendency of a Drilling Assembly in Real-Time;” U.S. Pat. No. Re. 35,790 to Psutanyk et al., titled “System for Drilling Deviated Boreholes;” U.K. Patent No. 2,369,685, titled “Method of Determining Trajectory in Borehole Drilling;” and U.K. Patent No. 2,370,361, titled “Borehole Survey Method and Apparatus.
Of course, in any drilling system utilizing magnetometer- and accelerometer-based sensor arrays to measure and control the trajectory of the drillstring, optimal reliable performance of such systems is necessarily dependent upon the accuracy of the sensor data that is provided from the down-hole sensors. Accordingly, it is generally understood that steps should preferably be taken to address the various factors that can adversely impact the accuracy or precision of the sensor data. Numerous such factors have been recognized in the prior art, and numerous approaches for addressing such factors have been proposed in the prior art.
For example, U.S. Pat. No. 5,806,194 to Rodney et al. proposes a method of correcting for the distorting effect of cross-axial magnetic interference on the readings of a well survey tool. Rodney et al. propose taking certain measurements of gravitational and cross axis magnetic fields at two or more axial locations in a well bore and using 11 these readings to statistically estimate the cross-axis interference. The Rodney et al. '194 patent is commonly assigned to the assignee of the present invention, and is hereby incorporated by reference herein in its entirety.
Numerous other teachings relating to accounting for certain types of error in survey tool magnetometer and accelerometer sensor readings are known in the prior art. See, e.g., U.S. Pat. No. 6,021,577 to Shiells et al., entitled “Borehole Surveying;” and U.S. Pat. No. 6,470,275 to Dubinsky, entitled “Adaptive Filtering with Reference Accelerometer for Cancellation of Tool-Mode Signals in MWD Applications.”
U.S. Pat. No. 5,321,893 to Engebretson proposes a technique intended to correct for fixed or induced magnetic fields in segments of a drillstring. According to Engebretson, a drillstring has an anomalous magnetization composed of both a fixed component resulting from permanently magnetized elements in the bottom hole assembly (“BHA”) and an “induced” component resulting from the interaction of soft magnetic materials with the Earth's magnetic field. Engebretson seeks to model the along-axis component and compensate for this error in computation of azimuthal direction independent of inclination and direction.
Those of ordinary skill in the art will appreciate that in addition to this interaction of magnetic materials in the BHA with the Earth's magnetic field giving rise to an “induced” magnetic field in the drillstring, there is another, separate electromagnetic mechanism by which a magnetic field may be “induced” in a drillstring. In particular, according to Faraday's Law, when a drillstring rotates in the Earth's magnetic field, electrical currents are induced along the drillstring. The conduction of these induced currents along the drillstring, in turn, generates a magnetic field orthogonal to the drillstring, which interferes with the measurement of Earth's magnetic field. Such a magnetic field resulting from induced currents in a drillstring is to be specifically distinguished from the excess component of magnetic field that appears in a permeable material when it is immersed in an ambient field, despite the fact that both magnetic fields are sometimes referred to as “induced” fields. For clarity, the former induced magnetic field shall be referred to herein as a “rotation-induced magnetic field” and the latter an “ambient-induced magnetic field.” These two types of “induced” fields are quite different, and techniques for modeling and/or compensating for one would not be 11 effective to do so for the other. For example, it is believed that the techniques proposed in the above-referenced Engebretson '893 patent would not be completely effective, and perhaps may be completely ineffective, in compensating for rotation-induced magnetic fields.
A simple one-dimensional analysis of the problem of rotation-induced magnetic fields arising due to induced currents in a rotating drillstring reveals that the interfering magnetic field will tend to screen out the cross-axial components of the Earth's magnetic field and rotate the cross-axial field as observed in the reference frame of the drillstring. Empirical analysis has shown that this can result in serious survey errors; hence, drillers oftentimes are forced to interrupt the rotation of the drillstring in order to obtain accurate borehole survey data. Drillers may, in fact, be unaware of the phenomenon of rotation-induced current induction, but may nevertheless stop rotation of the drillstring to stabilize the survey instruments and obtain the most accurate measurement possible. If means were known to eliminate the effects of vibration on inclinometers and magnetometers, it would still be necessary to stop rotation when making measurements due to rotation-induced currents during rotation.